The present invention generally relates to inkjet printers and more particularly to apparatus and methods for generating high quality images on a color inkjet printer.
Thermal inkjet hardcopy devices such as printers, large format plotters/printers, facsimile machines and copiers have gained wide acceptance. These hardcopy devices are described by W. J. Lloyd and H. T. Taub in xe2x80x9cInk Jet Devices,xe2x80x9d Chapter 13 of Output Hardcopy Devices (Ed. R. C. Durbeck and S. Sherr, San Diego: Academic Press, 1988) and U.S. Pat. Nos. 4,490,728 and 4,313,684. The basics of this technology are further disclosed in various articles in several editions of the Hewlett-Packard Journal [Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. 1 (February 1994)], incorporated herein by reference. Inkjet hardcopy devices produce high quality print, are compact and portable, and print quickly and quietly because only ink strikes the paper.
An inkjet printer forms a printed image by printing a pattern of individual dots at particular locations of an array defined for the printing medium. The locations are conveniently visualized as being small dots in a rectilinear array. The locations are sometimes xe2x80x9cdot locationsxe2x80x9d, xe2x80x9cdot positionsxe2x80x9d, or pixelsxe2x80x9d. Thus, the printing operation can be viewed as the filling of a pattern of dot locations with dots of ink.
Inkjet hardcopy devices print dots by ejecting very small drops of ink onto the print medium and typically include a movable carriage that supports one or more printheads each having ink ejecting nozzles. The carriage traverses over the surface of the print medium, and the nozzles are controlled to eject drops of ink at appropriate times pursuant to command of a microcomputer or other controller, wherein the timing of the application of the ink drops is intended to correspond to the pattern of pixels of the image being printed.
The typical inkjet printhead (i.e., the silicon substrate, structures built on the substrate, and connections to the substrate) uses liquid ink (i.e., dissolved colorants or pigments dispersed in a solvent). It has an array of precisely formed orifices or nozzles attached to a printhead substrate that incorporates an array of ink ejection chambers which receive liquid ink from the ink reservoir. Each chamber is located opposite the nozzle so ink can collect between it and the nozzle. The ejection of ink droplets is typically under the control of a microprocessor, the signals of which are conveyed by electrical traces to the ink ejection element. When electric printing pulses activate the ink ejection element, a small portion of the ink next to it vaporizes and ejects a drop of ink from the printhead. Properly arranged nozzles form a dot matrix pattern. Properly sequencing the operation of each nozzle causes characters or images to be printed upon the paper as the printhead moves past the paper.
The ink cartridge containing the nozzles is moved repeatedly across the width of the medium to be printed upon. At each of a designated number of increments of this movement across the medium, each of the nozzles is caused either to eject ink or to refrain from ejecting ink according to the program output of the controlling microprocessor. Each completed movement across the medium can print a swath approximately as wide as the number of nozzles arranged in a column of the ink cartridge multiplied times the distance between nozzle centers. After each such completed movement or swath the medium is moved forward the width of the swath, and the ink cartridge begins the next swath. By proper selection and timing of the signals, the desired print is obtained on the medium.
Color inkjet hardcopy devices commonly employ a plurality of print cartridges, usually two to four, mounted in the printer carriage to produce a full spectrum of colors. In a printer with four cartridges, each print cartridge can contain a different color ink, with the commonly used base colors being cyan, magenta, yellow, and black. In a printer with two cartridges, one cartridge can contain black ink with the other cartridge being a tri-compartment cartridge containing the base color cyan, magenta and yellow inks, or alternatively, two dual-compartment cartridges may be used to contain the four color inks. In addition, two tri-compartment cartridges may be used to contain six base color inks, for example, black, cyan, magenta, yellow, light cyan and light magenta. Further, other combinations can be employed depending on the number of different base color inks to be used.
The base colors are produced on the media by depositing a drop of the required color onto a dot location, while secondary or shaded colors are formed by depositing multiple drops of different base color inks onto the same or an adjacent dot location, with the overprinting of two or more base colors producing the secondary colors according to well established optical principles. In color printing, the various colored dots produced by each of the print cartridges are selectively overlapped to create crisp images composed of virtually any color of the visible spectrum. To create a single dot on paper having a color which requires a blend of two or more of the colors provided by different print cartridges, the nozzle plates on each of the cartridges must be precisely aligned so that a dot ejected from a selected nozzle in one cartridge overlaps a dot ejected from a corresponding nozzle in another cartridge.
The specific partial-inking pattern employed in each pass, and the way in which these different patterns add up to a single fully inked image, is known as a xe2x80x9cprintmode.xe2x80x9d The concept of printmodes is a useful and well-known technique of laying down in each pass of the printhead only a fraction of the total ink required in each section of the image, so that any areas left white in each pass are filled in by one or more later passes do not print all the required drops of all ink colors in all pixel locations in the swath in one single scan, or xe2x80x9cpassxe2x80x9d, of the printheads across the media. Rather, multiple scans are used to deposit the full amount of ink on the media, with the media being advanced after each pass by only a portion of the height of the printed swath. In this way, areas of the media can be printed in on more than one-pass. In a printer which uses such a xe2x80x9cmulti-passxe2x80x9d printmode, only a fraction of the total drops of ink needed to completely print each section of the image is laid down in each row of the printed medium by any single pass; areas left unprinted are filled in by one or more later passes.
The print quality produced from an inkjet device is dependent upon the reliability of its ink ejection elements. A multi-pass print mode can partially mitigate the impact of the malfunctioning ink ejection elements on the print quality because each pass uses a different nozzle to print a particular row of the image, multi-pass printing can compensate for nozzle defects. In addition, multi-pass print modes tend to control bleed, blocking and cockle by reducing the amount of liquid that is on the page at any given time.
Printmodes allow a trade-off between speed and image quality. For example, a printer""s draft mode provides the user with readable text as quickly as possible. Presentation, also known as best mode, is slow but produces the highest image quality. Normal mode is a compromise between draft and presentation modes. Printmodes allow the user to choose between these trade-offs. It also allows the printer to control several factors during printing that influence image quality, including: 1) the amount of ink placed on the media per dot location, 2) the speed with which the ink is placed, and, 3) the number of passes required to complete the image. Providing different printmodes to allow placing ink drops in multiple swaths can help with hiding nozzle defects. Different printmodes are also employed depending on the media type.
One-pass mode operation is used for increased throughput on plain paper. Use of this mode on other papers will result in too large of dots on coated papers, and ink coalescence on polyester media. In a one-pass mode, all dots to be fired on a given row of dots are placed on the medium in one swath of the printhead, and then the print medium is advanced into position for the next swath. A two-pass printmode is a print pattern wherein one-half of the dots available for a given row of available dots per swath are printed on each pass of the printhead, so two passes are needed to complete the printing for a given row. Similarly, a four-pass mode is a print pattern wherein one fourth of the dots for a given row are printed on each pass of the printhead. In a printmode of a certain number of passes, each pass should print, of all the ink drops to be printed, a fraction equal roughly to the reciprocal of the number of passes.
A printmode usually encompasses a description of a xe2x80x9cprintmask,xe2x80x9d or several printmasks, used in a repeated sequence and the number of passes required to reach xe2x80x9cfull density,xe2x80x9d and also the number of drops per pixel defining what is meant by full density. The pattern used in printing each nozzle section is known as xe2x80x9cprintmask.xe2x80x9d A printmask is a binary pattern that determines exactly which ink drops are printed in a given pass or, to put the same thing in another way, which passes are used to print each pixel. Thus, the printmask defines both the pass and the nozzle which will be used to print each pixel location, i.e., each row number and column number on the media. The printmask can be used to xe2x80x9cmix upxe2x80x9d the nozzles used, as between passes, in such a way as to reduce undesirable visible printing artifacts.
Previous printers have always used black and color printmasks with resolutions that are integer multiples of each other. This limits the ability to optimize ink saturation and firing frequency for black and color print cartridges because the black and color printmask dot grids are limited to integer multiples of each other. The present invention allows the number of drops of ink for black and dot grids to be non-integral multiples of each other. This allows greater flexibility in placing the optimum amount of ink on the print media for each color.
Accordingly, it would be advantageous to have greater flexibility in writing system design by not limiting the black and color printmask dot grids to integer multiples of each other. This would allow the number of drops of ink for black and color dot grids to be non-integral multiples of each other and would allow for greater flexibility in placing the optimum amount of ink on the print media for each color.
The present invention is a method of operating a printing system having a printing-medium advance direction and a transverse direction that is perpendicular to the printing-medium advance direction and a first plurality of ink drop generators and a second plurality of ink drop generators by moving the first plurality of ink drop generators along the transverse direction while ejecting first ink droplets onto the printing medium in a dot grid pattern having a first resolution in the transverse direction and moving the second plurality of ink drop generators along the transverse direction while ejecting second ink droplets onto the printing medium in a second dot grid pattern having a second resolution in the second resolution in the transverse direction. The first and second resolutions are non-integer multiples of each other.